A PEMFC uses a proton exchange membrane having a hydrogen ion exchange property as an electrolyte membrane. The PEMFC includes a pair of electrodes formed on opposite surfaces of the proton exchange membrane and a separator. The PEMFC generates electricity and heat through an electrochemical reaction between a fuel gas containing hydrogen and air containing oxygen. The PEMFC has an excellent output property and a quick start capability and can be operated at a relatively low temperature. Therefore, the PEMFC has been widely used in various applications such as a portable power source, an automotive power source, and a home cogeneration plant.
The electrodes used for the PEMFC include a catalyst layer containing a supported metal catalyst, such as platinum group metal, and a diffusion layer formed on an outer surface of the catalyst layer and having breathable and electron conduction properties. The diffusion layer is generally formed of a carbon paper or a carbon non-woven fabric. The assembly of the proton exchange membrane and the electrodes formed on opposite surfaces of the proton exchange membrane is referred to as “a membrane-electrode assembly (MEA)”. A conductive separator is installed on an outer side of the MEA to mechanically fix the electrodes and electrically interconnect adjacent MEAs. The assembly of the conductive separator, the proton exchange membrane, and the electrodes is referred to as “a unit cell”.
A fluid passage is formed on the separator contacting the MEA to supply a reactant gas to an electrode surface and to deliver surplus gas and a reaction by-product. The fluid passage may be separately prepared and installed on the separator. However, the fluid passage is generally provided in the form of a groove shape on a surface of the separator.
Particularly, cathode and anode separation plates of the fuel cell separator require electric conduction, gas-tightness, and corrosion-resistance characteristics. Therefore, in order to form the groove, a method that forms the groove by cutting a resin-impregnated graphite plate, a method that forms the groove by compression-forming carbon-compound powders, or a method that forms the groove by pressing a metal plate and coating a corrosion-resistance material on the metal plate has been used.
Further, the PEMFC includes a cooling unit along which a coolant flows and which is installed for one through three unit cells to dissipate heat generated by the operation thereof. The cooling unit includes a fuel gas separation plate having a first surface provided with a fuel gas passage and a second surface provided with a coolant passage, and an oxidizing gas separation plate having a first surface provided with an oxidizing gas passage and a second surface provided with a coolant passage. The fuel gas separation plate is assembled with the oxidizing gas separation plate such that the coolant passage surface (the second surface) of the fuel gas separation plate contacts the coolant passage surface (the second surface) of the oxidizing gas separation plate. Alternatively, the cooling unit may include a fuel gas separation plate having a first surface provided with a fuel gas passage and a second surface (an even surface) that is not provided with any passage, and an oxidizing gas separation plate having a first surface provided with an oxidizing gas passage and a second surface (a coolant passage surface) provided with a coolant passage. The fuel gas separation plate is assembled with the oxidizing gas separation plate such that the even surface (the second surface) of the fuel gas separation plate contacts the coolant passage surface (the second surface) of the oxidizing gas separation plate.
The separation plate of the PEMFC is provided with at least two through holes for each of the fuel gas, the oxidizing gas, and the coolant. Then, by connecting the inlet and outlet of the gas passage to the through holes, the reactant gas or coolant is supplied to the corresponding passage through one of the through holes and the surplus gas and reaction by-product or coolant is discharged through another one of the through holes.
In the PEMFC, a plurality of the separation plates are stacked with one another and thus the through holes of the separation plates form a single manifold. This is referred to as “an internal manifold type”. Instead of forming the through holes in the separator, a pipe for dispensing the gas or other structures may be installed on an outer surface of the separator. This is referred to as “an external manifold type”.
As described above, the conventional PEMFC includes a plurality of stacked unit cells each having the membrane-electrode assembly and the anode and cathode separation plates disposed on the respective opposite surfaces of the membrane-electrode assembly. The unit cells are coupled to each other by an appropriate compression force.
However, since the conventional PEMFC has a characteristic where a hydrogen ion conduction property thereof rises in proportion to an amount of moisture contained in the proton electrolyte membrane that is formed of a perfluorosulfonic acid-based material, the reactant gas, moisture of the proton electrolyte membrane, and heat must be properly controlled in order to obtain satisfactory performance thereof.
Particularly, when the PEMFC operates under a high load condition, the electrochemical reaction increases and thus the amount of moisture generated from the cathode side increases. This disturbs the supply of the reactant gas to the electrode. This phenomenon is referred to as “a flooding phenomenon”. In addition, since the current density of the conventional PEMFC increases, the supply speed of the reactant gas is lower than the electrochemical reaction speed. This causes an increase of the concentration polarization phenomenon of the electrode.
Therefore, in order to solve the flooding phenomenon or the increase of the concentration polarization phenomenon, a method for reducing the reactant gas utilization rate has been usually used. That is, according to the method for reducing the reactant gas utilization rate, a flow rate of the reactant gas supplied to the fuel cell increases to increase the supply speed of the reactant gas to the electrode, thereby reducing the concentration polarization. In addition, in order to reduce the reactant gas utilization rate, a pressure difference between the inlet and outlet of the passage of the separator increases to improve the moisture removal capability from the passage, thereby improving the flooding phenomenon.
Nevertheless, in the above-described methods, an amount of the reactant gas supplied is excessively greater than an amount of the reactant gas that is required for the actual reaction. Therefore, since the fuel and oxidizing agent are excessively consumed, the efficiency of the stack is deteriorated or the consumption of power for operating a compressor supplying the reactant gas increases.
Therefore, in order to solve the flooding phenomenon and the concentration polarization phenomenon, a method for increasing a channel length and reducing the number of channels of the passage formed on the separator has also been used. However, this method has a problem in that pressure loss occurs unnecessarily at a low load region where both of the flooding phenomenon and the concentration polarization phenomenon do not occur.